At present, it is known that OLED displays can be equipped with means for compensating the loss of luminescence due to ageing, whereby such compensation in part is carried out in view of the differential ageing of the individual pixels. The differential ageing of the individual pixels occurs due to the different drive levels of each pixel over the lifespan of the display. For example, if there is often a blue sky displayed at the top part of the device, the blue pixels in this part of the display will show ageing effects such as reduced luminescence and/or reduced performance faster that other pixels of the display. This is a problem that much less exists with LCD display devices to the same degree.
There are two types of compensation methods and systems known which address the problem of differential ageing of OLED display devices. The first method and system comprises the integration of a light sensor circuit in each individual pixel that acts as a feedback circuitry. The current can be increased depending upon this feedback signal to compensate for the loss of luminescence and/or performance. Obviously, the higher the current to drive the pixel for compensating the loss of performance due to ageing, the faster the pixel ages further so as the pixel reaches the end of its life the failure becomes more rapid. While this approach is very accurate it has severe drawbacks in terms of cost implications, scaling and reducing the size of the pixels for higher resolutions, and complicated drive and production processes.
A second method for detecting and compensating the differential ageing effect of OLED display devices is based on a “model” approach. By keeping track, e.g. in non-volatile storing, of how much each individual pixel was driven over the lifetime of the display device a prediction of the reduction in performance for each pixel can be made based on a model. This can be done by analysing the video content or by monitoring the on-current time of each pixel. The second method is representing a much cheaper and simple solution but its accuracy is heavily dependent on the quality of the model used. Environmental factors such as temperature and moisture during the time of use can not be taken into account. Therefore, in practice this second method does not show very accurate results and still some part of the differential ageing problem remains visible. Thus, this type of compensation would not be acceptable for display devices used in medical imaging.
From US 2008/0055209 A1 and US 2008/005210 A1 a method for reducing brightness uniformity variations in active matrix OLED displays employing amorphous silicon thin-film transistors during its actual use is known. The method relates to selecting a representative group of pixels which are preferred to be evenly distributed over the whole display and measuring the total representative current of all selected pixels in response to known image signals. Based on that measurement a correction value is derived from an estimated value of light emitting element performance in response to known image signals. Then, the corrected value is employed to correct the image signals for the changes in the output of the light emitting elements and to produce compensated image signals. The method is based on the measurement of total current for a group of pixels which has the drawback that only an estimation for the actual behaviour of the OLED pixels can be used depending on the measured current. Moreover, the method is concentrating on uniformity and brightness corrections especially for large scale displays and thus the selection of representative pixels has to be made with an even distribution over the whole display device. Differential ageing effects of the OLED pixels are not detected or compensated by this method.
WO 2008/019487 A1 discloses a system and method for determining a pixel capacitance in OLED pixels. As the pixel capacitance is correlated to a pixel age a current correction factor can be determined to compensate the pixel drive current and account for degradation of the pixel that results from the pixel ageing. However, the system includes means for reading the pixel capacitance in each pixel circuit. That again results in a complicated built showing the above mentioned drawbacks for the sensor based correction method. Moreover, the method can not include information about the past operation of the OLED pixels to compensate for the degradation.
Further, WO 01/63587 A3 describes a method and apparatus for calibrating OLED display devices and automatically compensating for loss in their efficiency over time. The disclosed method is representative for the above mentioned “model” approach and is based on measuring the driving current for each individual pixel and the corresponding light efficiency. On the basis of that data a second light efficiency is calculated for each pixel taking a special decay factor into account and the driving current is altered depending on a factor proportional to the ratio of the first and second light efficiencies. For calibration of the OLED display device a photodetector, such as a camera, is stepwise moved in front of the display from sub-area to sub-area of the display in order to measure the light output of the actual sub-area and compare the output with the light output of the foregoing sub-area. Like that, uniformity over the whole display is achieved.
The described model approach for compensating the ageing effects of the OLED pixels is solely based on the uniform prediction of degradation of the pixels put into the model as well as the measurement of the current. Thus, the compensation achieved is not as accurate as it is required for an application in medical imaging.